This invention relates to alumina ceramics. More particularly, the invention relates to an alumina substrate, having a very smooth surface and high density in an as-fired condition, useful as an insulating substrate in electronic devices.
2. Description of the Prior Art
Recently, ceramics, particularly alumina ceramics, have been widely used as substrate material in micro-electronic components with resulting improvements in high speed operation characteristics and high density mounting. The surface smoothness characteristic of this substrate is considered important, in addition to the electric, mechanical and thermal characteristics. Particularly in a thin film hybrid integrated circuit, smoothness of the substrate surface is very important. Some previously used ceramic substrates with smooth surfaces have been obtained by glazing. However, such glazing substrates are inferior in electrical and thermal characteristics and, in addition, are very expensive. Therefore, it is desired that a substrate be economically produced and have a surface smooth enough to allow circuit components and thin films of very fine pattern to be directly formed thereon, without glazing. Another method of forming a smooth surface on an alumina substrate involves grinding the substrate after it is fired. However, since the alumina ceramic is very hard, grinding is impractical. For this reason, it is desired that an alumina ceramic substrate be developed which can provide a sufficiently smooth surface, even in an as-fired condition.
In general, the manufacturing methods of alumina ceramics are classified into the following two methods; one is the dry press method wherein a mixture of alumina powder and binder is fired after being pressed mechanically; and the other is the wet casting method wherein an organic agent consisting of binder, plasticizers, deflocculents and solvents are added to the alumina powder and this combined material is formed into a slip. Then, a green sheet, obtained by casting this slip, is fired. Among these two methods, the dry press method is most likely to lose smoothness of the ceramic surface, and for that reason, the wet casting method is employed for obtaining an alumina ceramic substrate having a smooth surface.
An alumina substrate having a smooth surface is obtained by the following manufacturing process. Various kinds of metal oxide powders are added to high purity and fine alumina powder as the grain growth inhibitor, for obtaining a raw material powder. An organic agent consisting of binder, plasticizers, deflocculents and solvents is added. The combined material is then thoroughly mixed with a ball mill. Thus, the resulting slip contains a large amount of powder. The slip is then formed into a thin leather hard sheet on a material having a smooth surface, such as a polyester film, by using a so-called doctor-blade. This wet casting method is called the slip casting or tape casting method. Thereafter, the sheet is dried into a solvent-free green sheet. After removing an organic component in the green sheet, as required, by means of calcine, the green sheet is fired under air or hydrogen atmosphere conditions. The green sheet obtained by the above process prior to firing has a very smooth reflecting surface. However, it is degraded, due to the grain growth of the alumina during firing. Thus, control of the firing atmosphere conditions, firing temperature and firing time is required in order to prevent grain growth of alumina. For example, as described in the specification of the U.S. Pat. No. 3,698,923, the firing temperature is selected in that method to be as low as 1425.degree. to 1500.degree. C. Although an alumina substrate fired in such a low temperature has indeed considerable surface smoothness, it has the disadvantage of developing a large number of pores because of insufficient grain growth and therefore, suffers from a low density. Therefore, such a method makes it impossible to produce alumina ceramic substrates having both excellent thermal conductivity and dielectric characteristics.
In order to eliminate the above disadvantage, U.S. Pat. No. 3,854,965 proposes a method to add a small amount of chromium oxide (Cr.sub.2 O.sub.3) and magnesium oxide (MgO) as grain growth inhibitor to the high purity alumina powder, when manufacturing an alumina substrate by the wet casting method. In that patent, the raw sheet is fired at a temperature of 1600.degree. C. in a hydrogen atmosphere and an alumina substrate having a fine and excellently smooth surface is obtained. That temperature is higher than the firing temperature in manufacturing existing alumina substrates having super-smooth surfaces but lower than the firing temperature (1700.degree. C.) in manufacturing high density and hard alumina ceramics used for jigs. Generally, improvement in the surface smoothness characteristics of alumina substrates and high density characteristics are contradictory to each other and are largely dependent upon the firing temperature. The major advantage of the above method lies in solving such discrepancies by mixing the chromium oxide and magnesium oxide, and promoting high density of the alumina ceramics without sacrificing surface smoothness However, if the chromium oxide is added and the firing is performed in an atmosphere of air, conditions develop as in the case of the earlier cited method. The prevention of degradation of surface smoothness due to firing cannot be obtained and the alumina substrate becomes colored dark brown. For this reason, in the above method, it is necessary to perform the firing under a hydrogen ambient atmosphere and, as a result, it is difficult to economically manufacture the desired alumina substrate.
Moreover, in such a method, the desired amount of chromium oxide to be added is about 0.02 to 0.03 wt% against the alumina powder and this amount is by far less than that of the metal oxide (0.1 to 0.5 wt%) which is well known as a grain growth inhibitor. This minute amount of chromium oxide has little function as a grain growth inhibitor but has a special function to flatly orient the grown alumina grain. In order that such minute amount of chromium oxide be capable of producing such a result, it is necessary to use the alumina powder having a purity of 99.99% or better. Therefore, in the above method, very expensive alumina powder obtained by means of the non-Bayer process, such as Linda A marketed by Union Carbide Co., is required. Consequently, excellent surface smoothness cannot be obtained by the above method of U.S. Pat. No. 3,854,965 when the more economical alumina powder, with purity of about 99.5% and produced by the Bayer process is used.
Furthermore, in the above method, the formation of the ceramic substrate is accompanied by large shrinkage with sufficient grain growth for high density. As a result, a holding plate, which is mounted on the green sheet or the calcined substrate for the purpose of eliminating curvature of substrate at the time of firing, may cause damage to the substrate surface due to friction.